This application is based on patent application No. 2001-172969 filed on Jun. 7, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a nitride semiconductor stack and its semiconductor device, and more particularly to a semiconductor device such as a double heterojunction bipolar transistor and light emitting device using a nitride semiconductor stack.
2. Description of the Related Art
There arises band discontinuity at heterointerfaces between GaAs and AlGaAs, GaAs and InGaAs, and InGaAs and InP, which have been researched intensively.
FIG. 1 is a band diagram of a heterojunction of n-type InGaAs and n-type GaAs. In FIG. 1, the reference numeral 11 designates n-type InGaAs (including 10% In composition), and 12 designates n-type GaAs. Because of the band discontinuity in the heterostructure formed between the two semiconductors with small and large bandgaps, electrons cannot travel smoothly from the InGaAs with a smaller bandgap to the GaAs with a larger bandgap.
This is because the electrons are repelled by the discontinuity of the conduction band at the heterointerface 13 between the n-type InGaAs 11 and n-type GaAs 12. To eliminate the adverse effect of the band discontinuity, a graded layer is inserted between the two semiconductors so as to vary the bandgap gradually.
FIG. 2 is a band diagram of a structure including a graded layer between the n-type InGaAs with 10% In composition and the n-type GaAs. In FIG. 2, the reference numeral 21 designates the n-type InGaAs (including 10% In composition), 22 designates the n-type GaAs, and 23 designates the InGaAs graded layer. The In composition of the InGaAs in the graded layer 23 is varied from 10% to 0%, in which the InGaAs with 0% In composition corresponds to the GaAs. In this case, the electrons can move towards wider bandgap n-type GaAs smoothly without feeling the bandgap discontinuity. As for the double heterojunction bipolar transistor (DHBT) composed of the npn-type InGaAs and GaAs, the graded layer 23 is generally inserted between its base and collector (see, H. Ito and T. Ishibashi, Jpn. J. Appl. Phys. 25 (1986) L421).
In this case, the graded layer 23 is composed of InGaAs, in which the In composition varies gradually from 8% to 0% in a range of 30 nm. Although the graded layer is doped with n-type impurities of about 2xc3x971017 cmxe2x88x923, heavy doping of more than 1xc3x971018 cmxe2x88x923 is not performed. This is because the heavy doping presents a problem of reducing the breakdown voltage between the base and collector.
Examples of InGaAs/GaAs or InGaAsN/GaAs DHBTs reported up to now all disclose the graded layer of 30 nm thick, and the n-type impurity concentrations of 1xc3x971017 (cmxe2x88x923) and 3xc3x971016 (cmxe2x88x923) (H. Ito and T. Ishibashi, Jpn. J. Appl. Phys. 25 (1986) L421, and N. Y. Li et al. Electron. Lett. 36 (2000)). A thinner graded layer will present the following problems: (1) The composition control becomes difficult; (2) The electrons do not feel the graded layer because the spread of the wave function of electrons is about an order of 10 nm. On the other hand, the collectors range from 200 to 500 nm thick in almost all examples fabricated. When the thickness of the graded layer is not negligible compared with the thickness of the collector, there arises another problem of long collector transit time, which increases its response time and deteriorates the high frequency characteristics. For these reasons, the conventional reports are supposed to use rather thin graded layers of 30 nm thick.
In the nitride semiconductor heterojunction, space charges are brought about in the heterointerface because of piezoelectric effect or spontaneous polarization. The amount of the space charges is approximately proportional to a lattice constant difference between the two layers constituting the heterojunction. In other words, an increasing lattice constant difference between the two layers will increase the piezoelectric charges. Generally, an increasing lattice constant difference between the two layers will also increase the energy difference of the bandgap.
FIG. 3 is a diagram showing a heterojunction of InGaN and GaN. In FIG. 3, the reference numeral 31 designates InGaN, 32 designates GaN, 33 designates a heterointerface and 34 designates space charges in the heterointerface. Since the InGaN 31 is present on the surface side, negative space charges 34 occur in the heterointerface 33 and the band is modulated by the space charges 34. In contrast with this, when the GaN 32 is present on the surface side, positive space charges are produced in the heterointerface 33.
It is expected that the space charges will be generated in the nitride semiconductor graded layer which is formed with varying the composition of the constituent elements. Currently, no idea is proposed that when the graded layer is formed using the nitride semiconductor, the space charges are generated uniformly in the graded layer. However, as will be described in the examples in accordance with the present invention, it was found that the space charges were generated in the graded layer.
As for the DHBTs, a graded structure is interposed between the base and collector with varying the bandgap gradually. Because of the space charges generated in the graded layer using the nitride semiconductor, the electrons injected from the emitter cannot reach the collector through the base. Thus, the collector current cannot be increased, and the current gain cannot be increased.
In addition, there arises another problem in that it is difficult for the light emitting devices to inject electrons or holes into the active layer because of the space charges in the graded layer.
The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide a nitride semiconductor stack and its semiconductor device with a transistor structure capable of achieving high current gain by enabling the electrons injected from the emitter to reach the collector by solving the problem of being unable to increase the collector current in the DHBTs because of the space charges in the graded layer, which are generated by the piezoelectric effect or spontaneous polarization.
Another object of the present invention is to provide a nitride semiconductor stack and its semiconductor device capable of implementing a light emitting device structure that can reduce the voltage needed for the light emission and increase the luminous efficiency by efficiently injecting electrons or holes into the active layer by solving the problem in that it is difficult to inject the electrons or holes into the active layer because of the space charges in the graded layer, which are generated by the piezoelectric effect or spontaneous polarization.
To accomplish the objects, according to a first aspect of the present invention, there is provided a nitride semiconductor stack composed of a nitride compound semiconductor comprising: a nitride semiconductor layer that has a structure in which a bandgap is gradually varied from a substrate side to a surface side, that is doped with at least one of n-type and p-type impurities with a high concentration of at least 1xc3x971018 cmxe2x88x923, and has a thickness of 10-100 nm, and that is interposed between an n-type layer at the substrate side and a p-type layer on the surface side.
Here, the bandgap may be gradually decreased, and the nitride semiconductor layer may be doped with the n-type impurities.
The bandgap may be gradually increased, and the nitride semiconductor layer may be doped with the p-type impurities.
According to a second aspect of the present invention, there is provided a semiconductor device consisting of a double heterojunction bipolar transistor that is fabricated using the foregoing nitride semiconductor stack, and includes a collector with a bandgap wider than a bandgap of a base, wherein the semiconductor device is an npn-type heterojunction bipolar transistor in which the base is disposed closer to a surface side of a substrate than the collector, and the nitride semiconductor layer is interposed between the base and the collector.
According to a third aspect of the present invention, there is provided a semiconductor device consisting of a light emitting device that is fabricated using the foregoing nitride semiconductor stack, and that includes from the substrate side to the surface side an n-type layer with a bandgap wider than a bandgap of an active layer, the active layer and a p-type layer with a bandgap wider than the bandgap of the active layer, the semiconductor device comprising: a layer that is interposed between the n-type layer and the active layer or between the active layer and the p-type layer, and has a bandgap gradually varied from the substrate side to the surface side, and that is doped with at least one of n-type and p-type impurities with a high concentration of at least 1xc3x971018 cmxe2x88x923, and has a thickness of 10-100 nm.
Here, the layer may be interposed between the n-type layer and the active layer, have a structure in which the bandgap is gradually reduced from the substrate side to the surface side, and be doped with the n-type impurities.
The layer may be interposed between the active layer and the p-type layer, have a structure in which the bandgap is gradually increased from the substrate side to the surface side, and be doped with the p-type impurities.
The layer may be interposed between the n-type layer and the active layer, have a structure in which the bandgap is gradually reduced from the substrate side to the surface side, and be doped with the n-type impurities, and the semiconductor device may further comprise a layer that is interposed between the active layer and the p-type layer, may have a structure in which the bandgap is gradually increased from the substrate side to the surface side, and be doped with the p-type impurities.
With the foregoing structures, the graded layer composed of the nitride semiconductor brings about space charges because of the piezoelectric effect or spontaneous polarization. Since the space charges modulate the band, they form a barrier to electrons or holes moving in a direction perpendicular to the heterointerface. The heavy doping of impurities for canceling out the space charges will prevent the modulation of the band. As a result, the effect is eliminated of the space charges generated by the piezoelectric effect or spontaneous polarization, thereby enabling the electrons or holes to travel smoothly in the direction perpendicular to the heterointerface.
Heterojunction bipolar transistors (HBTs) used as a radio-frequency power amplifier sometimes utilize a double heterostructure in which the bandgap of the collector is wider than the bandgap of the base to obtain a high output. The present invention relates to a structure for achieving high current gain in a double heterojunction bipolar transistor (DHBT) using a nitride semiconductor. It also relates to a structure for achieving high luminous efficiency by reducing the voltage for emission in the light emitting device using the nitride semiconductor.
The most important characteristic of the present invention is to dope the graded layer with impurities of a high concentration equal to or greater than 1xc3x971018 cmxe2x88x923 in order to cancel out the space charges generated in the graded layer using the nitride semiconductor. The present invention differs from the conventional techniques in that the graded layer is doped with impurities of the type different from the type of the space charges generated in the graded layer in the nitride semiconductor, and that the impurity concentration is at least 1xc3x971018 cmxe2x88x923.
According to the present invention, high concentration impurities are doped to cancel out the effect of the space charges which are considered to be generated in the graded layer composed of the nitride semiconductor because of the piezoelectric effect or spontaneous polarization effect. As a result, in the npn-type DHBT having the graded layer interposed between the base and collector, the electrons injected from the emitter can pass through the base and reach the collector. Thus, the DHBT has an advantage of being able to increase the collector current, thereby achieving high current gain. As for the light emitting device in accordance with the present invention, the electrons or holes can be injected into the active layer efficiently. As a result, the voltage required for the emission can be reduced, and increase the luminous efficiency.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.